In electrophotographic document copier machines, copies of documents or other subjects are produced by creating an image of the subject on a photoreceptive surface, developing the image and then fusing the image to copy material. In machines which utilize plain bond copy paper or other ordinary image receiving material not specially coated, the electrophotographic process is of the transfer type where a photoreceptive material is placed around a rotating drum or arranged as a belt to be driven by a system of rollers. In the typical transfer process, photoreceptive material is passed under a stationary charge generating station to place a relatively uniform electrostatic charge, to a potential which is usually several hundred volts, across the entirety of the photoreceptive surface. Next, the photoreceptor is moved to an imaging station where it receives light rays reflected from the document to be copied. Since white areas of the original document reflect large amounts of light, the photoreceptive material is discharged in white areas to relatively low levels while the dark areas continue to contain high voltage levels even after exposure. In that manner, the photoreceptive material is caused to bear a charge pattern which corresponds to the printing, shading, etc. present on the original document and is therefore, an electrostatic image of that document.
Electrophotographic machines may also be organized to provide a printing function where the image on the photoreceptive surface results from character generation rather than from an optical review of an original document. Character generation may be produced, for example, by driving a light generating source from information held in digital memory. The generating source may be a laser gun, an array of light-emitting diodes, light modulators, etc. which direct light rays to the photoreceptor and cause it to bear a charge pattern which is an image of the information used to drive the generating source.
After producing an image on the photoreceptor, the next step in the process is to move the image to a developing station where developing material called toner is placed on the image. This material may be in the form of a black powder which carries a charge opposite in polarity to the charge pattern on the photoreceptor. Because of the attraction of the oppositely charged toner, it adheres to the surface of the photoreceptor in proportions related to the shading of the original. Thus, black character printing should receive heavy toner deposits, white background areas should receive none, and gray or otherwise shaded half-tone character portions of the original should receive intermediate amounts.
The developed image is moved from the developer to a transfer station where a copy receiving material, usually paper, is juxtaposed to the developed image on the photoreceptor. A charge is placed on the back-side of the copy paper so that when the paper is stripped from the photoreceptor, the toner material is held on the paper and removed from the photoreceptor. Unfortunately, the transfer operation seldom transfers 100% of the toner from the receptor to the copy paper.
Toner remaining on the photoreceptor after transfer is called residual toner.
The remaining process steps call for permanently bonding the transferred toner material to the copy paper and cleaning the residual toner left on the photoreceptor so that it can be reused for subsequent copy production.
In the cleaning step, it is customary to pass the photoreceptor under a preclean charge generating station to neutralize the charged areas on the photoreceptor. The photoreceptor may also be moved under an erase lamp to discharge any remaining charge. In that manner, the residual toner is no longer held by electrostatic attraction to the photoreceptive surface and thus it can be more easily removed at a cleaning station.
In order to avoid overburdening the cleaning station, it is customary to remove all charge present on the photoreceptive surface outside of the image area prior to the development step. This is usually done by using an interimage erase lamp to discharge photoreceptive material between the trailing edge of one image and the leading edge of the next. Also, erase lamps are used to erase charge along the edges of the photoreceptor outside of the image area. For example, if the original document is 8.5.times.11 inches in size, and if a full sized reproduction is desired, the dimensions of the image on the photoreceptor will also be 8.5.times.11 inches. The interimage and erase lamps remove charge outside of the 8.5.times.11-inch image area.
A common variation on the above-described process used in many electrophotographic machines involves the use of specially prepared paper where the copy paper itself carries a coating of photosensitive material. By utilizing that technique, the image is electrostatically painted directly on the copy paper. The copy paper is sent through a developer and then to a fuser for permanent bonding. Machines of this type avoid the residual toner problem and therefore there is no need for cleaning stations, erase lamps, preclean generating coronas, etc. However, the resulting copy paper with its special photosensitive coating is much more expensive than plain bond copy paper and the special coating is considered to detract from the resulting product. As a consequence, coated paper machines are usually favored only for low volume applications or where quality product is not essential.
In addition to the fundamental mechanisms used for producing a copy or print, modern electrophotographic machines have been developed with many features which are designed to ease the difficulty of using the machines. For example, semiautomatic (SADF) and automatic (ADF) document feed devices, including a variety which recirculates the originals, ease the entry of documents to be copied. Collators are often added to the base machine so that collated sets of copies can be automatically produced. Many machines have a duplex function so that copies can be produced on both sides of the copy sheet. Other features add to machine versatility such as the production of copies which are a reduced or magnified version of the original document. Other features improve copy quality such as mechanisms for controlling the concentration of toner in machines which utilize a carrier/toner development mix. Many modern electrophotographic machines are controlled by microprocessors rather than by hardwired analog or digital logic. The use of microprocessors has enabled the addition of many new innovative functions at low cost such as, for example, error logs and automatic diagnostic capabilities to ease troubleshooting and improve maintenance. Microprocessor routines have also aided in the establishment of a degree of "artificial intelligence" to anticipate the needs of the machine user in document feed operations, collate, and other areas. Additionally, microprocessors have made economical the addition of innovative functions such as the provision of separator sheets between different sets of copies within a collator.
As may be appreciated from the above, the basic electrophotographic machine involves the interaction of several important subsystems to produce a copy sheet or print and several other subsystems which control copy quality or which provide convenience functions. In addition, modern copier machines have been developed to run at high speeds so that, at any one time, several copy sheets can be located along the copy paper path from the image-receiving station to the exit station. These sheets can all be copies of the same original document, but in many cases these sheets will bear an image of more than one original. As a result, if a paper jam occurs necessitating a machine shutdown and a clearing of all sheets in the copy paper path, copies of several different originals might be thrown away thereby causing a need to recopy these same several different originals in order to recover from the jam. U.S. Pat. No. 4,229,100 to Travis (IBM) discloses such a machine and describes a system which utilizes counts for precisely recovering from loss of copy sheets due to a jam or other stoppage conditions. In the described system, the maximum number of originals for which recopies might need to be made is limited to three and the system identifies the number of originals, one, two, or three which need to be recopied. A similar technique is used on the commercially available IBM Series III Copier/Duplicator. However, the system described by Travis, as well as commercially available machines, require the insertion of makeup originals by the machine user, and, after jam recovery, require operator intervention to restart the job interrupted by the work stoppage.